The present invention relates to a process for the continuous production of axially parallel fiber reinforced synthetic plastic shapes. More particularly, the invention relates to production of fiber-reinforced round rods having large cross sections and high quality for use in high-voltage technology, wherein fibers are drawn from spools, passed through perforated plates, impregnated in suitable installations with a resin, shaped by means of nozzles, and hardened into solid shapes in a hardening furnace. For use in high-voltage technology, shaped synthetic plastic bodies of this type must be able to simultaneously withstand both high electrical stresses and large mechanical forces. Mechanical strength of the synthetic plastic shapes is obtained in particular by means of the axially parallel orientation of the reinforcing fibers, which ordinarily amount to from 40 to 75 volume percent of the shaped body. Glass fibers are used for the most part, but other organic or inorganic fibers may be employed.
Processes of this general type are known. An installation is described in German Auslegeschrift No. 1 264 742, in which the hardening line comprises three successive furnaces. A nozzle is located in front of each furnace, and the diameters of the nozzles decrease toward the end of the line. The nozzles are intended to shape the cross section of the shaped body. Excess resin is squeezed out from between the fibers during the process. However, with thick-walled shapes only the outer range of the fiber matrix is compressed, and the resin seeks an outlet transversely to the axis of the shaped body, by breaking out laterally from the body. The result is a poor distribution of the glass fibers, particularly in the case of thicker round rods. Furthermore, during the solidification of the shaped body, the nozzles tear the sensitive surfaces, whereby cracks are produced again.
It is a fundamental principle that in high-voltage technology only insulating parts which do not contain any internal inhomogeneities such as air bubbles, cavities and cracks, can be used. When a high voltage is applied to the shaped body, such inhomogeneities lead to a partial internal discharge, which could lead to an extended electrical breakdown. Such a breakdown may also result in the mechanical failure of the insulating body.
Inhomogeneities of this type may originate in various stages of the production process, for example in an inadequate saturation of the fibers whereby air bubble are drawn into the shaped body. On the other hand, during shaping by means of draw nozzles, the impregnating resin may be caused to bake onto the wall of the shape. This results in cracking on the surface of the shape. Particularly when highly active impregnating resins are used or when thick walled shapes are formed, there is a risk that the reaction heat generated cannot be removed rapidly enough, which leads to shrinkage cracking in the shaped body.
Another problem arises in connection with the distribution of the fibers over the cross section of the shaped body. Irregular fiber distributions lead to internal stresses and thus to low mechanical properties of the shaped bodies and favor generation of shrinkage cracking inside the shaped body. Especially in the manufacture of round rods having diameters greater than 3 cm, the distribution of fibers plays an important role. Similarly, high tensile stresses must be avoided during the manufacturing process, since the impregnating resin tends to shrink three-dimensionally during hardening. High tensile stresses in the direction of the axis of the shaped body interfere with the axial shrinkage of the impregnating resin, again causing the shape to crack.